As fast food giant McDonald's struggles to retain customers domestically and abroad, it reported its worst monthly comparable sales decreases in the U.S. and Europe since early 2003, according to a new note from Janney Montgomery Scott.

"Holy moley!" Janney analyst Mark Kalinowski said in a phone interview. "This one is certainly not going to be talked about for decades to come in a positive way."

In the third quarter, global comparable sales dived 3.3 percent.The fast food giant was expected to see its global same store sales shrink by 3 percent with U.S. sales falling by 2.9 percent and those in the Asian Pacific Middle East Africa unit dropping by 10.6 percent, according to a forecast from Consensus Metrix.

To spark a turnaround it its U.S. unit, McDonald's has been ramping up its transparency efforts and highlighting its food quality. Abroad, McDonald's sales have seen an impact after a scandal involving one of its suppliers in China, which it has since suspended.

Analysts had expected McDonald's to deliver earnings of $1.37 per share on $7.18 billion in revenue, according to Thomson Reuters.

"McDonald's third quarter results reflect a significant decline versus a year ago, with our business and financial performance pressured by a variety of factors—from a higher effective tax rate, to unusual events in the operating environments in APMEA and Europe, to under-performance in the U.S., our largest geographic segment," McDonald's President and Chief Executive Officer Don Thompson said in a statement.